Assertion (A): The leaves of cacti are reduced to spines. Reason (R): Spines reduce water loss by transpiration. MCQ with Answer and Explanation

Explanation:
Self-pollination (autogamy) occurs within the same flower. Geitonogamy is pollination between different flowers of the same plant, functionally cross-pollination. Xenogamy is between different plants. Self-pollination leads to genetic uniformity.

Explanation:
The Sinoatrial (SA) node, located in the right atrium, is the natural pacemaker because its cells are autorhythmic; they can spontaneously generate action potentials without external neural input. Crucially, the SA node has the highest intrinsic rate of depolarization (about 70-80 beats per minute) compared to other autorhythmic tissues like the AV node or Purkinje fibers. Therefore, it sets the pace for the entire heart, overriding slower pacemakers.

Explanation:
Diabetes insipidus results from ADH deficiency (or resistance), causing polyuria, polydipsia, and dilute urine. Diabetes mellitus is insulin-related, Addison's is adrenal insufficiency, Cushing's is excess cortisol.